Quote from Ofcom (via MSE, admittedly a few years ago so link has been removed):

"ITV operates an independently developed winner selector tool under licence from a third party.
This is known as 'Winner Selector’ and is a market leading solution used for managing many
competitions across the UK. The Winner Selector works by taking in all entries to a competition
from each entry route, and assigning a unique random number to every entry. This is done
independently of the original entry method, and assures that each entry has the same chance of
being selected, irrespective of the method of entry."